This project will explore the neural processing of social cues from potential mates and the neuroendocrine mechanisms by which these social cues modulate reproductive physiology. Social cues, particularly from individuals of the opposite sex, play an important role in regulating reproduction. However, the role of social attachment (i.e. pair bond) between potential mates has largely been ignored in considering the effect of intersexual social cues on reproductive physiology. Vasopressin and oxytocin are important in pair-bond formation induced by mating, but less is known about their role in the formation of pair bonds prior to mating. Given the importance of these neuropeptides in the formation of social attachments and for sexual behavior, they may be mechanistically important in transducing intersexual social cues to the reproductive axis. The proposed research will test whether (1) social attachment modulates the effect of a potential mate on reproductive physiology, (2) vasopressin and oxytocin homologues (vasotocin and mesotocin) affect the likelihood that pair bonds will form prior to mating, and (3) test whether vasotocin and mesotocin are involved in transducing intersexual cues to the gonadotropin-releasing hormone (GnRH) system. The project will use two approaches. First, measures of reproductive function (GnRH immunoreactivity, plasma LH levels, and gonad size), as well as vasotocin and mesotocin immunoreactivity and receptor binding, will be compared among individuals from pairs that differ in their degree of social attachment. Second, these neuropeptides and their antagonists will be administered to assess their effects on the formation of partner preferences prior to mating and on the GnRH system. Thus far, research into reproductive physiology and the neurobiology of affiliation has been conducted in relative isolation. The proposed work will begin to connect research in these two areas, in order to gain significant insights into how intersexual social interactions affect the brain and reproductive physiology. Human social relationships affect both mental and reproductive health. This study investigates the neural mechanisms by which social attachments are formed and by which they influence reproductive function in an animal model with important similarities to humans: mate choice selectivity, pre-mating pair bond formation, and long-lasting pair bonds. LAY SUMMARY: Social relationships influence both mental and reproductive health in humans. This study investigates neural mechanisms by which social attachments are formed and by which social interactions influence reproduction.